This invention pertains to the art of transport refrigeration units, and in particular to a solid-state defrost control system for such a unit.
Various defrost control arrangements have been used in connection with transport refrigeration units. These include time-controlled defrost intervals and demand defrost control, as well as combinations of these.
The aim of this invention is to provide a solid-state defrost control system, relying principally upon a time-controlled defrost interval, and which has at least the following features. Sensing of the unit evaporator coil temperature is done with an electronic sensor, and the control system will continue to function even in the event of an open or a short of the sensor. Defrost time intervals of several different intervals can be selected easily, and a maximum defrost time is established. The system can include and be interfaced with a manual defrost mode, and an air switch backup mode, with protection in the case of a sticking air switch. The system has thermostat set point monitoring to determine when the timer is to be ON or OFF depending upon whether the set point for the space to be conditioned is below or above a pre-established temperature. The operation of the timer continues, regardless of the thermostat set point, if the sensor were to fail in either an open or short condition. When used with a refrigeration unit in which the drive for the unit is capable of ON/OFF operation, the timer will stop and retain time when on the OFF cycle. The system provides time and temperature integration of defrost intervals when the thermostat set point is above a pre-established temperature and the device will accumulate time only when the evaporator coil temperature is less than that pre-established temperature, and will retain and hold the time when the evaporator coil temperature is above that pre-established temperature.